Strong, Shape-Memory Lignocellulosic Aerogel via Wood Cell Wall Nanoscale Reassembly

Polymer shape-memory aerogels (PSMAs) are prospects in various fields of application ranging from aerospace to biomedicine, as advanced thermal insulators, actuators, or sensors. However, the fabrication of PSMAs with good mechanical performance is challenging and is currently dominated by fossil-based polymers. In this work, strong, shape-memory bio-aerogels with high specific surface areas (up to 220 m2/g) and low radial thermal conductivity (0.042 W/mK) were prepared through a one-step treatment of native wood using an ionic liquid mixture of [MTBD]+[MMP]−/DMSO. The aerogel showed similar chemical composition similar to native wood. Nanoscale spatial rearrangement of wood biopolymers in the cell wall and lumen was achieved, resulting in flexible hydrogels, offering design freedom for subsequent aerogels with intricate geometries. Shape-memory function under stimuli of water was reported. The chemical composition and distribution, morphology, and mechanical performance of the aerogel were carefully studied using confocal Raman spectroscopy, AFM, SAXS/WAXS, NMR, digital image correlation, etc. With its simplicity, sustainability, and the broad range of applicability, the methodology developed for nanoscale reassembly of wood is an advancement for the design of biobased shape-memory aerogels.

. Mass balance of the wood treatment Figure S1. BJH pore-size distribution of freeze-dried NW-Aerogel   Figure S1. BJH pore-size distribution of freeze-dried NW-aerogel Figure S2. Left-hand side, ambient dried sample from water having a twisted shape. Right-hand side reswollen sample from water, returned back to its original flat shape Figure S3. Larger area confocal Raman image. From the left, lignin signal is scanned and is clearly seen with intense signal form the CC and CML with decreasing signal towards the lumen. Middle image shows the cellulose signal, where the secondary cell walls show strongest signal followed by the fibrillated networks of the lumen space which is clearly seen. Furthest right is the combined image of lignin and cellulose signal. Figure S4. Confocal Raman images of a thin out-diffused cell wall. From the left, the cellulose signal is shown, clearly showing the strong cellulose signal from secondary cell wall and fibrillated networks within the lumen. It is apparent that the formed cellulose networks come from the cell walls. In the middle, lignin signal from the CML is shown, giving a notion that most of the remaining cell wall is from the lignin-rich CML. The right-hand image shows the merged signal which accentuates the message of cell walls mostly consisting of the lignin-rich CML after dissolution/regeneration of wood. Figure S5. Left-hand side, native wood sample at 100x magnification and 10x. Right-hand side, NW-Aerogel sample at 100x and 10x using FCA staining. Blue represents carbohydrates and red lignin.

DIC Details:
DIC technique is used to measure the strain distribution on native wood and aerogel samples. Specifically, mirror-assisted multi-view digital image correlation (MV-DIC) 7 (see figure S10) that can retrieve the panoramic strain distribution is adopted to measure the highly inhomogeneous and complicated deformation behavior. Similar to the other compressive tests, DIC measurements were performed also during a radial compression in a conditioned room of 23 °C and 50% relative humidity. Cubic samples with nominal dimension of 10 x 5 x 5 mm 3 (longitudinal × radial × tangential) were compressed with preset constant strain rate of 10%/min by an Instron E1000 compression machine equipped with a 10 kN load cell.
Prior to the experiments, the sample surfaces were pretreated by slight polish, followed by the decoration of thin white base coat and black random speckles. These samples were compressed by cylindrical steel heads with diameter of 10 mm. Image series were collected by a stereo-DIC system during the loading progress to record the surface evolvement resulted from compression. To be specific, a Blue-X-Focus blue light source were fixed in front of the sample to provide uniform and constant light on the sample surface.
Two Basler acA4096-30um cameras with resolution of 2168×4096 were mounted on a tripod. The stereo-DIC system needs to be calibrated in advance for strain measurement. A planar calibration target with 9×9 circular feature points is used. The constant space between the node points is 3 mm. More than 25 calibration image pairs were recorded by moving the calibration target to diverse poses and positions.
The intrinsic and extrinsic parameters of the stereo-DIC system can be determined by processing these calibration image pairs. Panoramic surface deformation was retrieved by processing the recorded image pairs, see the image video 5 and video 6. Four regions of interest (Fig. S10, surface 1-4), including the two sample surfaces reflected by each mirror and the speckle patterns on the two mirrors, were specified for DIC matching. The subset size and step size between calculation points were designated to be 31 × 31 and 5 pixels, respectively. The strain was estimated from a small window size of 7 ×7 points. Aided by the reconstructed 3D shape of the two mirrors, the measurement results of the two sample surfaces, which were reflected by the two mirrors, were reflected to their real locations in front of the mirrors, leading to the panoramic strain distribution.